Video merge feature, default avatar, video-redirects table, misc UI tweaks

Bundled pre-existing changes on the branch:
- Video merge: controller, service, admin merge modal, and a repair
  console command; new video_redirects table (with source_folder)
  so merged/moved videos keep resolving under their old keys.
- Default avatar SVG for users without a profile picture.
- Assorted view tweaks across profile, video actions/comments/insights,
  admin layout, video-details, and the app layout.
- Small touch-ups in SuperAdminController, User model, NasSyncService.

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>

app/Models/User.php

@@ -168,7 +168,7 @@ class User extends Authenticatable implements MustVerifyEmail
             return route('media.avatar', $this->avatar).'?v='.($this->updated_at?->timestamp ?? '0');
         }
 
-        return 'https://i.pravatar.cc/150?u='.$this->id;
+        return asset('images/default-avatar.svg');
     }
 
     public function getBannerUrlAttribute(): ?string